7. Expression vs Statement

Articles

• Function Expressions vs Function Declarations — Paul Wilkins
• JavaScript Function — Declaration vs Expression — Ravi Roshan
• Function Declarations vs. Function Expressions — Mandeep Singh
• Function Declarations vs. Function Expressions — Anguls Croll

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Expressions vs Statement

Function declarations are hoisted, but function expressions aren't. Precisely, variable assigned to function expressions are hoisted, but it's uninitialized, so we can't use it before we assgin it.

Expressions

After function expression has stored in a variable, variable can be used as a function, which means that it is invoked by variable's name

let sum = function(num1, num2){
  return num1 + num2;
};

console.log(sum(1,2));

When to use function expressions

• Closure

function checkValid(input) {
  const password = "password123";
  return function() {
    if(input === password) {
      console.log("verified!");
    } else {
      console.log("wrong!");
    }
  }
}

checkValid("password123")(); // verified!
checkValid("password1234")(); // wrong!
console.log(password); // ReferenceError: password is not defined

In this snippet, function expression is used for closure that is closing over variable password. Although, we cannot access to this variable in global scope, we can use the inner function inside checkValid function which can still access to password. By using closure, we can hide the password, but can still check if the input is valid.

• Passing as arguments

function checkSum(num1, num2, callback) {
  if(callback(num1, num2) == num1 + num2) {
    return "It's sum function!";
  } else {
    return "Error";
  }
}

const sum = function(num1, num2) {
  return num1 + num2;
}

console.log(checkSum(2,3,sum)); // It's a sum function!

Function checkSum checks if the given function is a sum function. By using function expression, we made function sum and it's given as a parameter to checkSum function.

• IIFE
  IIFE helps preventing functions and variables from affecting the global scope.

const checkValid = ((input) => {
  const password = "password123";
  return function(input) {
    if(input === password) {
      console.log("verified!");
    } else {
      console.log("wrong!");
    }
  }
})();


checkValid("password123"); // verified!
checkValid("password1234"); // wrong!